European Patent No. EP 0 403 588 A1 describes a system in which an infrared light beam is directed onto a door and the infrared light beam reflected by the door is utilized for detecting an unauthorized intrusion. In this case, the system consumes a considerable amount of energy and is not optimal for an independent operation in which it is supplied with electrical energy only by a normal battery.
U.S. Pat. No. 8,253,563 B2 describes multiple sensors and a microcontroller, which, together, form a system for detecting a forcible intrusion at a door on the basis of statistically ascertained sensor signals. The disadvantage in this case is that physical variables which describe the position and the movement of the door are not taken into account in the detection of the forcible intrusion at the door.
In addition, in European Patent No. EP 1 652 159 A2, a gyroscope, a MEMS acceleration sensor, a piezoelectric acceleration sensor and a magnetic field sensor are utilized. In this case, the principle for detecting a door movement is based on audio signals. An extensive and expensive signal processing unit is necessary in order to achieve this, since audio signals must be generated and evaluated. This complex unit also requires a great deal of energy to operate.